Icons-only navigations are all around in many applications and services. It’s just something that works. The only time I can recall it troubled me was with Facebook’s live video tab that appeared in their mobile app. “What is it?”. The question did not only apply to the button but to the functionality itself as well.
The examples used for the pencil icon seemed pretty straightforward because there’s context. And in most cases it’s the context that explains the action, not a label under it.
Too much information isn’t a good UX decision either. It’s mostly about where to draw the line. Google and Apple diverge in some points, but both seem to draw the line fairly enough.